There is a distinction to be made between getting a Seat Reservation and getting a train ticket. Seat Reservations can be made 90 days out - and you do not need a ticket in hand to book those. Train tickets - you can only buy 60 days out. Now I do not remember if you plan to have a rail pass or if you are buying tickets but since you want specific trains on specific dates it is worth seeing if you can buy a SuperSaver ticket for the journey. These can be very cheap BUT require that you take the train at the time specified. Miss the train and there is no refund. worth looking at though...

Thank you Hambagahle for pointing this out- looking at my options now for train passes etc.

OK, I'm late to the party and am sorry to arrive after a protracted discussion about itinerary options... But, after travelling on the Bernina railway, why not take the train from Tirano to lake Como (perhaps Colico or Bellano), by boat to Como, and then by train again to Locarno (or elsewhere in Ticino)? That way you get to see beautiful lake Como (and part of lake Lugano from the train to Locarno), and it's not much more complicated or expensive than taking the special bus from Tirano to Locarno. You could spend a night in Bellagio, or Chiavenna, or Como itself.

To head back to Switzerland after riding the Bernina Express southbound you can also take the train from Tirano to Chiavenna (with one change, I believe) and the bus from there to St. Moritz (via Maloja). From St Moritz you can take the Glacier Express to Andermatt (along the most scenic parts of the route, all the way along the Albula railway and over the Oberalp pass), a local train to nearby Göschenen, and the Treno Gottardo to Ticino. If you want to get to Ticino fast, this is not the best option, of course.

Well, mbrevitas, we are staying with the route as I described above ( just cannot re-think this trip at this point ; but I do appreciate your input.
However, I do like the interesting options you offered to get to Locarno. I think several of you have offered the train/boat option.
I'm in the process of transportation schedules, so this is good timing!
Thank you

The boat which mbrevitas is referring to leaves Colico at 14.58 and arrives at Como at 18.49.
Connecting train from Poschiavo: Poschiavo dp 12.23 - Tirano ar 13.00, change train, Tirano dp 13.08 - Colico ar 14.14.
The boat pier of Colico can be seen from the railway station. Distance 500 metres/0.3 mile
The distance between the boat pier of Como and Como San Giovanni railway station is 1 km (0.6 mile). You may take a citybus or a taxi.
Connecting train to Locarno: Como SG dp 19.23 - Locarno ar 20.35.
But be aware that there are also boats starting from Locarno (Lago Maggiore, up tpo Isola Bella and Stresa) and from Lugano (Lago di Lugano) up to the cog wheel railway to the view point Monte Generoso, to the Swissminiatur open air museum, to the Cantine di Gandria Smuggler museum, to the very scenic villages of Morcote and Gandria and to the Brusino - Serpiano cableway leading to the Unesco word heritage site area Monte San Giorgio.

Monte Generoso

Monte Generoso railway

The current timetable (PDF at 02_NLC_Colico_Como_I2021V.pdf) doesn't have that boat connection; there's one at 13:55, one at 16:05, and one at 17:30 (plus two in the early morning). This timetable is valid until 26 March only, though; perhaps the 14:58 one is there in the summer...

Thank you all !!

mbrevitas: the departs you mention are by hydrofoils where you can't sit outside. Not worth the hassle nor the money.
As Jenny will travel in April, I posted the spring timetable (not confirmed yet, but there will hardly be any change) for boats (not for hydrofoils!).